**5. Proprieties and volumetric compensation of the gases from anaerobic process**

The physical and chemical proprieties of biogas, affect the choice of technology used for clean-up and combustion; therefore, knowledge of these proprieties is useful for optimization biogas utilization. Since biogas contains primarily methane and carbon dioxide, this section is focused on their respective physical characteristics (Table 4). Because others components (nitrogen, hydrogen sulfide, traces organics), are present in relatively small quantities are not considered in the table. The magnitude of CH4 and CO2 varies greatly and depends on the composition of the organic material digested in the wastewater.


a: pure gas given at 77 ºF and 101.3 kPa; b: sublimes; c: Air at 101.3 kPa and 15.54 °C

Table 4. Physical constants of methane and carbon dioxide

The volumetric measurement of biogas must compensate the pressure and temperature differences. The equation 10, illustrate a simple method of gas volume compensation for a saturated gas taking into account the adjustment by pressure and temperature (Salisbury, 1950):

$$\text{V}\_{\text{s}} = \text{V} \ast 17.626 \ast \frac{(\text{H} - \text{A})}{(459.6 + \text{T})} \tag{10}$$

where *V* is the observed volume, *Vs* volume at standard conditions (60°F and 30 inches Hg), *H* is absolute gas pressure (inches Hg), *A* water vapor pressure (inches Hg), and *T* temperature of gas (°F). Pure methane at standard temperature and pressure has a lower heating value of 912 BTU/ft3 (34 kJ/L). Typical biogas of 65% methane has a heating value of approximately 600 BTU/ft3 (22.36 kJ/L), since only the methane portion will burn, approximate equivalents of biogas to others fuels are presented in the table 5.


Table 5. Equivalents of biogas others fuels (Palmer, 1981)
